Various methods have been proposed for recording apparatuses that record a text, image, and the like on recording paper or sheet-like recording media such as a film. A typical example is an inkjet method of forming a text and image on a recording medium by applying a recording material (color material) on a recording medium.
Recording apparatuses of the inkjet type (to be referred to as inkjet recording apparatuses) are classified into a serial apparatus that scans a recording head in a recording region vertically and horizontally, and a line-head apparatus that scans, in only one direction, a recording head in which recording elements are arranged fully widthwise in a recording region.
The serial inkjet recording apparatus forms an image on a recording medium by alternately repeating main scanning of moving a carriage supporting a recording head with respect to a recording medium while discharging ink from recording elements, and sub-scanning of carrying a recording medium in a direction perpendicular to the main scanning direction. The serial inkjet recording apparatus generally employs a recording method called multipass recording, and forms an image stepwise by scanning the same region on a recording medium a plurality of number of times.
FIGS. 1A to 1C are schematic views for explaining multipass recording.
FIG. 1A shows the state of a recording medium on which a color material discharged from a recording head 104 is applied by the first scanning. Dots 101 are recorded by the first scanning not to overlap each other.
FIG. 1B shows the state of the recording medium on which a color material is applied after the second scanning. Dots 102 are recorded by the second scanning while overlapping the dots 101.
FIG. 1C shows the state of the recording medium on which a color material is applied after the third scanning. Dots 103 are recorded by the third scanning while overlapping the dots 102.
Recording is completed by the two, first and second scan operations in the upper half region of the recording medium shown in FIGS. 1A to 1C, and by the two, second and third scan operations in the lower half region. The positions of dots to be recorded by each scanning are determined by the AND of image data of an image to be formed and a mask pattern (binary data indicating dot recording positions and non-recording positions in scanning). In the example shown in FIGS. 1A to 1C, the recording ratio of each scanning is 50%. The recording ratio becomes 100% by two scan operations (so-called two passes), forming an image.
In this state example, the dots 101 do not overlap each other, the dots 101 and 102 overlap each other, and the dots 102 and 103 overlap each other. However, overlapping of dots are determined by the dot size, the recording ratio in each scanning, and the recording resolution.
A recording medium is carried between scan operations in multipass recording, so recording elements which record dots in a given region change every scanning. Even if the discharge characteristics of recording elements vary, multipass recording can distribute the influence of variations and make it less conspicuous on a formed image. The density of a recorded portion sometimes changes at the joint between scan operations owing to variations in the recording medium carrying amount, generating a line due to a change in density at the joint. However, multipass recording can obscure the line at the joint in a formed image.
Variations in the discharge characteristics of recording elements and the carrying amount are image deterioration factors arising from the manufacturing process and precision. Thus, multipass recording is an important technique for maintaining the image quality in a serial inkjet recording apparatus.
As an ink for inkjet recording apparatuses, dye ink using a water-soluble dye as a color material is popular. The color material dissolved in a solvent in the dye ink, whose main component is water, easily permeates into the fiber of a recording medium. This makes it easy to maintain the surface shape of a recording medium even after recording an image, and the gloss of the recording medium is kept as that of the image. In other words, a combination of a recording medium and dye ink that has exceptional gloss provides a glossy image. The user of the inkjet recording apparatus using the dye ink can obtain an image with the desired glossiness by selecting a recording medium that has the preferred glossiness.
In contrast, higher light resistance and higher water resistance are requested of printed materials. The dye molecule of the color material of the dye ink is dissolved by light, and the colors of an image readily fade. When a printed material gets wet, the dye molecule permeated in the fiber is dissolved in water, and the image smears readily. That is, a material printed with the dye ink generally suffers low light resistance and water resistance.
To solve poor light resistance and water resistance of a material printed with the dye ink, pigment ink using a pigment as a color material has been developed these days. Unlike the dye ink in which the dye exists as a molecule in a solvent, the color material of the pigment ink exists in a solvent as a particle several ten nm to several hundred nm in diameter. The color material particle of the pigment ink is larger than the dye molecule of the dye ink, and can provide a printed material excellent in light resistance. Since the pigment is insoluble in water, the pigment ink is superior to the dye ink even in water resistance.
In recording with the pigment ink, the pigment particle hardly permeates into a recording medium and piles on the surface of the recording medium. As a result, the fine shape (smoothness) of the image surface differs between a recording region where the pigment ink is applied and a non-recording region where it is not applied.
The amount of color material used changes depending on the density and color of an image formed on a recording medium, and the area by which the pigment covers a recording medium changes. Since the pigment and recording medium have different reflectances, the gloss changes upon a change of the area by which the pigment covers a recording medium.
In this manner, even the glossiness changes depending on the density and color of an image in recording using the pigment ink. As a result, one image has regions different in gloss, that is, a glossy region observed to be glossy and a mat region observed not to be glossy. A change of gloss in one image is recognized as “heterogeneity of glossiness”. The heterogeneity of glossiness is often recognized as poor quality particularly on a printed photo image.
To solve this problem, for example, inventions in Japanese Patent Laid-Open Nos. 2006-272934 and 2007-276482 propose methods using a clear ink containing no color material. More specifically, Japanese Patent Laid-Open No. 2006-272934 discloses a technique of adjusting the discharge amounts of color and clear inks to uniform, in the entire unrecorded region of a recording medium, the amount of resinous component derived from pigment ink per unit area in the unrecorded region. Japanese Patent Laid-Open No. 2007-276482 discloses a technique of adjusting the discharge amounts of colored and clear inks to substantially uniform glossiness in an unrecorded region.
The techniques in Japanese Patent Laid-Open Nos. 2006-272934 and 2007-276482 can uniform glossiness. However, depending on the type of ink and printing conditions, specular reflection light from a recorded surface is colorized differently based on the image tone and the color difference may visually stand out. The colorization of specular reflection light is conspicuous especially in a region recorded with a bright ink such as yellow ink or clear ink. The study by the present inventors has revealed that this phenomenon is interference (structural color) by a thin film and the thickness of a color material layer determines the color of specular reflection light.
FIG. 2 is a schematic view for explaining the principle of interference by a thin film.
When an image is formed with pigment ink, part of light is reflected by the surface of a color material layer 1301 formed on a recording medium 1302, and another passes through the color material layer 1301 and is reflected by the surface of the recording medium 1302. These light beams differ in optical path length by a thickness 1303 of the color material layer 1301 and interfere with each other, colorizing specular reflection light. The optical path difference between the two reflection light beams depends on the thickness 1303, so the color of specular reflection light depends on the thickness 1303 of the color material layer 1301.
The tone is expressed by changing the applying amount of colored ink. The change of the amount being applied leads to a change of the thickness 1303 of the color material layer 1301. It will be understood that the color of specular reflection light changes depending on the tone.